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What Psi Will Support Combustion?


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#1 Alc ONLINE  

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Posted May 28, 2012 - 08:34 PM

As a general rule I'm guessing there is a minimum psi that will support combustion when starting an engine . I'm thinking 90psi would be a the bottom end . Even with an engine that has a compression release it has to have SOME to get it fired off . Thanks Al

#2 caseguy OFFLINE  

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Posted May 28, 2012 - 08:50 PM

I have had engines read as low as 45 and still run well but be lacking in power to do any kind of work and die quickly under load. In fact, before I redid the rings and valves in my Case SC, the best of the four cylinders was 75 and the worst was 45. It would still run like a champ, but lacked power which prompted the compression check in the first place.
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#3 MH81 ONLINE  

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Posted May 28, 2012 - 09:03 PM

I think most of the Teccy's I've checked with compression release read around 55-60 when cranking.
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#4 chris m OFFLINE  

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Posted May 28, 2012 - 09:04 PM

:iagree: :yeah_that: :ditto: :nothingtoadd: :laughingteeth:
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#5 middleageddeere OFFLINE  

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Posted May 28, 2012 - 09:25 PM

I was under the assumpution that around 60. I thought 90 was good...

#6 olcowhand OFFLINE  

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Posted May 29, 2012 - 05:30 AM

Onan engines do NOT have a compression release system. Like new they have around 115psi. They will run decent at 75psi, but 85 to 90psi seems to be the lowest to get good power.
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#7 Alc ONLINE  

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Posted May 29, 2012 - 05:56 AM

Thanks all , what brought this question up was the DB with a 2 1/2 HP B&S on it won't start, first thing it didn't have spark so ,cleaned the points and rest them and mag. to get an OK spark and tried new a new plug ,seems like fuel to the plug because had to clean them a few time trying . compression seemed a little low but if the info I read on it it was only less the 6:1 new :( Tried blowing compressed air into the spark plug hole , ( pretty tough keeping it TDC ) intake was leaking a little . Did compression test and at first thought thinks where good at 90psi until I realized that the gauge didn't return to zero stayed at 30psi :confuse: Time to check it with another gauge but guessing a new one is in order . Taking a guess then it's around 60psi but will recheck with another gauge . Also in between all that pulled the head to make sure the valves open and closed on their own and other then carbon didn't look all that bad . Al

#8 GTTinkerer OFFLINE  

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Posted May 29, 2012 - 06:19 AM

Al,
I don't know if a B&S of that vintage would have compression release or not. Have you checked the valve clearance? It doesn't have to be off by much to cause a noticeable amount of compression pressure loss. The way I have checked valve seating when in a rush is to turn the engine over until one of the valves is all the way open and then try to spin the closed valve by hand and then reverse it to check the other valve seating

Bill.
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#9 MH81 ONLINE  

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Posted May 29, 2012 - 06:58 AM

Once you get your gauge working, take a reading, then squirt a little 30 weight in there and take it again.

Ive found If it goes up noticeably, it's probably rings, if not, valves.


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#10 Alc ONLINE  

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Posted May 29, 2012 - 10:36 AM

Bill I should have metioned that I caecked the valve clearence too , both where close enough not to even mess with ( that means they would have been farther off if I tried to adjust them lol ) Alan , I'll have to bring a compression gauge home from work tonight to recheck , Al

#11 TUDOR OFFLINE  

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Posted May 29, 2012 - 05:16 PM

Less than atmospheric pressure will support combustion. The problem with an internal combustion engine is to keep the expansion of gases that are the product of combustion confined to the cylinder swept area to get some work out of it. Rings, valves, and gaskets must work together to keep the higher pressure confined to the combustion zone for this to happen.

Even with zero compression on an engine, the gas will burn...... slowly. The pressure just escapes as fast as the gases expand and it won't retain enough pressure to turn the engine. Enough of the resulting product of combustion, carbon dioxide, remains in the cylinder to snuff out the fire. The result is that most of the gas doesn't have enough time to burn off and the engine gets flooded on the second compression stroke.

I recall reading somewhere the minimum compression needed for combustion to generate enough power just to turn an engine, but I can't remember what it was, other than less than I thought.

That is background info only and doesn't really contribute to the primary purpose of this discussion. It only addresses the original question.

Edited by TUDOR, May 29, 2012 - 05:18 PM.

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#12 Amigatec OFFLINE  

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Posted May 29, 2012 - 07:10 PM

I'll bet it didn't have much when it was new, I've never checked mine, I know it doesn't have much, but it starts and runs good.

#13 olcowhand OFFLINE  

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Posted May 29, 2012 - 07:34 PM

Onan engines require more psi than many older engines. I have an old Lauson that I bet doesn't build 50psi and she runs, or did a few years back. It's mothballed at the moment.

#14 Alc ONLINE  

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Posted May 29, 2012 - 07:52 PM

Well , reset the gauge to zero by using my air compressor /regulator got it within 5 psi so called it close enough. Went back and got 70 psi , checked the 5 hp B&S on the other DB that runs and that only had 60 psi :confuse: Started all over again and rechecked the spark , now I don't have any :confuse: , tried another plug and still no go, then pulled some more and it has a nice spark :confuse: Can't figure what's going on with the spark that comes and goes . Condenser ? Would appreciate any ideas , thanks , Al

#15 MH81 ONLINE  

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Posted May 29, 2012 - 07:56 PM

Condenser would be my guess too. Either that or points.
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